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Weather Chart Information
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Station observation plot  Cloud Cover Chart Cloud Symbols The unit of cloud cover is oktas, or eighths of the sky. When the sky is completely covered by cloud (overcast), the cloud cover is eight oktas. When the sky is clear the reading is zero oktas. Few. This indicates 1 or 2 oktas of cloud. Scattered. This indicates 3 or 4 oktas of cloud. Broken. This indicates 5 to 7 oktas of cloud. Overcast. This indicates 8 oktas (solid cloud cover). Information on Clouds from Aust. BoM Ceiling The terms ceiling and cloud ceiling are defined as the height-above-ground level of the lowest broken or overcast layer. The level displayed in the Station Plot (above) is in hundreds of feet of the lowest cloud base, so L35 = 3,500 feet, L6 = 600 feet, L0 means the cloud's on the ground. Wind barbs Wind barbs are simply a conventient way to represent both wind speed and direction in a compact graphical form.  Wind barbs point in the direction "from" which the wind is blowing.  Wind speed is given here in the units of "knots" (knts). A "Knot" is a nautical mile per hour. (1 Knot = 1.15 MPH, = 1.9 Kph) Each short barb represents 5 knots, each long barb 10 knots and Pennants 50 knots. A long barb and a short barb is 15 knots. Simply by adding the value of each barb together (10 knots + 5 knots = 15 knots). If only a station circle is plotted, the winds are calm. Isobars Isobars (iso = equal, bar = pressure) are lines which join points of equal atmospheric pressure. Some isobars have numbers on them showing this value in hectoPascals (hPa). Generally the closer they are the stronger the wind. Highs or Anticyclones When isobars enclose an area of high pressure this is called a high or anticyclone and its centre is labelled on a weather chart with an H. In the Southern Hemisphere, winds go around highs in an anticlockwise direction. An intensifying high has a rising central pressure, while a weakening high has a falling central pressure. Lows or Depressions When isobars enclose an area of low pressure this is called a 'low' or 'depression' and its centre is labelled on a weather chart with an L. In the Southern Hemisphere, winds go around lows in a clockwise direction. If there are two or more centres the depression is said to be complex. If the central pressure is rising the low is said to be filling or weakening, if the central pressure is falling the low is said to be intensifying or deepening. Tropical Cyclones Tropical cyclones are also areas of low pressure and the winds go around them in a clockwise direction (in the Southern Hemisphere). Unlike depressions though, they form only in the tropics and have quite a different character. A tropical low is only considered to be a tropical cyclone when the wind near its centre is gale force (34 - 47 knots) or stronger. Because of their destructive potential, tropical cyclones are given names. |
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